1. Field of the Invention
The present invention relates to a heating apparatus for cooking such as microwave ovens in which a frozen food (as a target food item to be cooked) placed on a turntable in a cavity as a heating chamber is thawed and heated by irradiating a microwave of a high frequency to the food item.
2. Description of the Related Art
In many conventional heating apparatus for cooking such as microwave ovens to control a heating process by detecting a temperature of a target food to be heated by using an infrared ray (IR) sensor of a non-contact type, it is impossible to accurately detect a temperature of an area which is smaller in size than the detection field of the IR sensor (or the field of view of the IR sensor), because the detection field of the IR sensor is so elliptical in shape that the IR sensor outputs the average value of a temperature distribution in the detection field of the IR sensor.
For example, when the detection field of the IR sensor overlaps only with a boundary area (or the end part) of a target food to be detected, in other words, when a part in the detection field of the IR sensor overlaps with the boundary area of the food, the temperatures of both the target food and a background area in which there is no target food are averaged. In this case, it is impossible to obtain an accurate temperature of the food. In order to avoid this conventional drawback, when the detection field of the IR sensor is set to narrow, the resolution of the IR sensor becomes high and the occurrence that the detection field of the IR sensor overlaps with the boundary area of the food can be small, but it is impossible to avoid any occurrence that the detection field of the IR sensor overlaps with the boundary area of the food completely. Thus, even if the detection field of the IR sensor is set to narrow, the detected temperature in the above case also has an inaccurate value. In addition, to narrow the detection field of the IR sensor greatly causes to decrease the sensitivity of the IR sensor and to decrease the function of noise immunity, so that the accuracy of the detection by the IR sensor becomes low.
If the number of IR elements forming the IR sensor is only one, it is impossible to set the detection field of the IR sensor into a narrow area because it is necessary to keep a widely detectable area in a heating chamber of the heating apparatus for cooking.
Furthermore, even if the IR sensor comprises a plurality of the IR elements in order to detect a plurality of detection areas in the heating chamber in the heating apparatus for cooking, it is possible to set the detection field of each IR element to narrow, but, there still remains the drawback that it is impossible to detect: the temperature of the boundary area (or the end part) of the target food accurately.
During a thawing process in practical cooking, for example, the temperature of a localized area becomes increased by uneven cooking of the target food, uneven heating of the target food, and the like, the part of the target food becomes a half cooked state (namely, a half-boiled state) or a boiled state. This case often occurs at a boundary area of the target food. The conventional heating apparatus for cooking such as microwave ovens cannot solve this drawback because the conventional heating apparatus for cooking cannot detect accurately the increasing of the temperature of the boundary area of the target food, so that it cannot prevent: to cause the half-cooking state or the uneven state of the target food.